PostCSS CSS Variables

  

PostCSS plugin to transform CSS Custom Properties (CSS variables) syntax into a static representation. This plugin provides a future-proof way of using most of CSS variables features, including selector cascading with some caveats, because this can only see the CSS, not the potentially dynamic HTML and DOM the CSS is applied to.

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``
npm install postcss postcss-css-variables --save-dev
`

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- Code Playground
- Usage
- Syntax - Defining Custom Properties with --* - Using Variables/Custom Properties with var()
- Features - At-rules like @media, @support, etc. - Pseudo-classes and Elements - Nested Rules
- Why? - Interoperability - Differences from postcss-custom-properties
- Caveats
- Options
- Quick Reference/Notes
- Testing
- Changelog

Code Playground

Try it in the playground and see what you think! Just add some CSS and see to see the final transformed/compiled CSS. You can try anything here in the playground, too.

Usage

_For more general PostCSS usage, look here._

`js
var postcss = require("postcss");
var cssvariables = require("postcss-css-variables");

var fs = require("fs");

var mycss = fs.readFileSync("input.css", "utf8");

// Process your CSS with postcss-css-variables
var output = postcss([cssvariables(/options/)]).process(mycss).css;

console.log(output);
`

Syntax

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A custom property is any property whose name starts with two dashes --. A property must be in a rule.

_Note: :root is nothing more than the selector for the root DOM node. Any other selector like .class, #id, or even #foo ~ .bar > span.baz works._

`css
:root {
--foo-width: 100px;
--foo-bg-color: rgba(255, 0, 0, 0.85);
}

.foo {
--foo-width: 100px;
--foo-bg-color: rgba(255, 0, 0, 0.85);
}
`

Custom properties can be declared multiple times, but like variable scope in other languages, only the most specific one takes precedence.

`css
:root {
--some-color: red;
}

.foo {
/ red /
color: var(--some-color);
}

.bar {
--some-color: blue;
/ blue /
color: var(--some-color);
}

.bar:hover {
--some-color: green;
/ Automatically gets a color: green; declaration because we --some-color used within scope elsewhere /
}
`

_W3C Draft: CSS Custom Properties for Cascading Variables, section 2_

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`css
.foo {
width: var(--foo-width);
/ You can even provide a fallback /
background: var(--foo-bg-color, #ff0000);
}
`

_W3C Draft: CSS Custom Properties for Cascading Variables, section 3_

Features

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It's perfectly okay to declare CSS variables inside media queries and the like. It'll work just as you would expect.

`css
:root {
--width: 100px;
}

@media (max-width: 1000px) {
:root {
--width: 200px;
}
}

.box {
width: var(--width);
}
`

Will be transformed to:

`css
.box {
width: 100px;
}

@media (max-width: 1000px) {
.box {
width: 200px;
}
}
`

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Psuedo-classes are also dealt with correctly, because it's easy to statically determine.

`css
.foo {
--foo-color: red;
color: var(--foo-color);
}

.foo:hover {
--foo-color: green;
}
`

Will be transformed to:

`css
.foo {
color: red;
}

.foo:hover {
color: green;
}
`

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This pairs very well with postcss-nested or postcss-nesting, adding support for nested rules. For either, you must put the plugin before postcss-css-variables in the plugin stack so that the & references are expanded first (postcss-css-variables doesn't understand them). For example, with postcss-nested, your PostCSS setup would look like this:

`js
var postcss = require("postcss");
var cssvariables = require("postcss-css-variables");
var nested = require("postcss-nested");

var fs = require("fs");

var mycss = fs.readFileSync("input.css", "utf8");

var output = postcss([
// Flatten/unnest rules
nested,
// Then process any CSS variables
cssvariables(/options/)
]).process(mycss).css;

console.log(output);
`

For a simple example with nesting:

`css
.box-foo {
--some-width: 150px;
width: var(--some-width);

.box-bar {
width: var(--some-width);
}
}
`

With also postcss-nesting, this will be transformed to:

`css
.box-foo {
width: 150px;
}

.box-foo .box-bar {
width: 150px;
}
`

For a more complex example with a media query:

`css
:root {
--some-width: 150px;
}

.box-foo {
width: var(--some-width);

.box-bar {
width: var(--some-width);
}
}

@media (max-width: 800px) {
.box-foo {
--some-width: 300px;
}
}
`

Will be transformed to:

`css
.box-foo {
width: 150px;
}

.box-foo .box-bar {
width: 150px;
}

@media (max-width: 800px) {
.box-foo {
width: 300px;
}

.box-foo .box-bar {
width: 300px;
}
}
`

Why?

This plugin was spawned out of a discussion on the cssnext repo and a personal need.

There is another similar plugin available, postcss-custom-properties, although it restricts itself much more than this plugin, preferring partial spec conformance. This plugin has the same capabilities but also adds imperfect feature support which stem from not being able to know what the DOM will look like when you compile your CSS. We instead look at the explicit structure of your CSS selectors.

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Putting postcss-css-variables in place of postcss-custom-properties should work out of the box.

In postcss-custom-properties, CSS variable declarations are specifically restricted to the :root selector.

In postcss-css-variables, this is not the case and they may be declared inside any rule with whatever selector. The variables are substituted based on statically known CSS selector inheritance.

Here's a quick overview of the differences:

- CSS variables may be declared in any selector like .foo or .foo .bar:hover, and is not limited to just :root
- CSS variables may be declared in @media, @support, and other at-rules.
- CSS variables may be declared in :hover and other psuedo-classes, which get expanded properly.
- Variables in nested rules can be deduced with the help of postcss-nested or postcss-nesting.

Continue to the next section to see where some of these might be unsafe to do. There are reasons behind the ethos of why the other plugin, postcss-custom-properties, is very limited in what it supports, due to differing opinions on what is okay to support.

Caveats

When you declare a CSS variable inside one selector, but consume it in another, this does make an unsafe assumption about it which can be non-conforming in certain edge cases. Here is an example where these limitations result in non-conforming behavior.

Note the nested markup below. We only know about the DOM's inheritance from your CSS selectors. If you want nest multiple times, you need to be explicit about it in your CSS which isn't necessary with browser that natively support CSS variables. See the innermost